The quadrupole ion trap (QIT) was first disclosed in the year 1952 in a paper by Paul, et al. This paper disclosed the QIT and the disclosure of a slightly different device which was called a quadrupole mass spectrometer (QMS). The quadrupole mass spectrometer was very different from all earlier mass spectrometers because it did not require the use of a magnet and because it employed radio frequency fields for enabling the separation of ions, i.e. performing mass analysis. Mass spectrometers are devices for making precise determination of the constituents of a material by providing separations of all the different masses in a sample according to their mass to charge ratio. The material to be analyzed is first dissociated/fragmented into ions which are charged atoms or molecularly bound groups of atoms.
The principle of the quadrupole mass spectrometer (QMS) relies on the fact within a specifically shaped structure radio frequency (RF) fields can be made to interact with a charged ion so that the resultant force on certain of the ions is a restoring force thereby causing those particles to oscillate about some reference position. In the quadrupole mass spectrometer, four long parallel electrodes, each having highly a precise hyperbolic cross sections, are connected together electrically. Both dc voltage, U, and an RF voltage, V.sub.0 cos .omega.t, can be applied. When an ion is introduced or generated within the spectrometer, if the parameters of the quadrupole are appropriate to maintain the oscillation of those ions, such ions would travel with a constant velocity down the central axis of the electrodes at a constant velocity. Parameters of operation could be adjusted so that ions of selected mass to charge ratio, m/e, could be made to remain stable in the direction of travel while all other ions would be ejected from the axis. This QMS was capable of maintaining restoration forces in two directions only, so it became known as a transmission mass filter. The other device described in the above mentioned Paul, et al. paper has become known as the quadrupole ion trao (QIT). The QIT is capable of restoring forces on selected ions in all three directions. This is the reason that it is called a trap. Ions so trapped can be retained for relatively long periods of time which supports separation of masses and enables various important scientific experiments and industrial testing which can not be as conveniently accomplished in other spectrometers.
The QIT was only of laboratory interest until recent years when relatively convenient techniques evolved for use of the QIT in a mass spectrometer application. Specifically, methods are known for creating ions of an unknown sample after the sample was introduced into the QIT, and adjusting the QIT parameters so that it stores only a selectable range of ions from the sample within the QIT. Then by linearly changing, i.e., scanning, one of the QIT parameters it became possible to cause consecutive values of m/e of the stored ions to become successively unstable. The final step in a mass spectrometer was to sequentially pass the separated ions which had become unstable into a detector. The detected ion current signal intensity, as a function of the scan parameter, is the mass spectrum of the trapped ions.
U. S. Pat. No. 4,736,101 describes a quadrupole technique for performing an experiment called MS/MS. In U. S. Pat. No. 4,736,101, MS/MS is described as the steps of forming and storing ions having a range of masses in an ion trap, mass selecting among them to select an ion of particular mass to be studied (parent ion), disassociating the parent ion by collisions, and analyzing, i.e. separating and ejecting the fragments (daughter ions) to obtain a mass spectrum of the daughter ions. To isolate an ion for purposes of MS/MS the '101 patent discloses a method of scanning (ramping up) the RF trapping field voltage according to known equations to eject ions having atomic mass up to the m/e of ion of interest. Then, the RF trapping field voltage is lowered and the ions remaining are disassociated by collision. Finally, the RF trapping voltage is scanned up again and a mass spectrogram of the ejected daughter ions is obtained. One technique for obtaining collision induced disassociation (CID) to obtain daughter ions is to employ a second fixed frequency generator connected to the end plates of the QIT which frequency is at the calculated secular frequency of the retained ion being investigated. The secular frequency is the frequency in which the ion is periodically, physically moving within the RF trapping field.
The '101 patent also discloses use of a supplementary RF field voltage applied to the end cap electrodes of a QIT containing daughter ions while the RF trapping field is being scanned as a means of successively ejecting increasing mass ions to obtain a spectrum. In this instance, the patent employs a reduced maximum magnitude of the RF trapping field voltage.
The difficulty with the technique of the '101 patent is that after the ionization step, the parent ion, m(p), is selected for MS/MS using the so called mass instability method. This is where one of the quadrupole parameters, i.e. the RF field voltage, is varied to move the ions having M/e outside the range of interest into the instability region, i.e. q.sub.z &gt;0.908. In the '101 patent this was accomplished by ramping up the RF trapping field voltage to cause those ions having M/e less than the selected parent ion, m(p), to be ejected. Ions of mass greater than m(p) are retained in the trap. The voltage level of the RF trapping field is then lowered and CID accomplished. This means that ions having greater than the M/e of the selected m(p) were present during CID. These ions can cause interference and/or unwanted reactions or daughter ions.
The problem of incomplete isolation in MS/MS of the parent m(p) ion is addressed in U. S. Pat. No. 4,749,860. In this prior patent, a second, supplemental RF field is applied to the end caps. The frequency of this supplemental RF field corresponds to the secular frequency of a specific ion having a M/e value which is one M/e unit greater than the selected parent ion, i.e. m(p)+1. The '860 patent applied this supplement RF field to the end caps simultaneously with the application of the ramping of the voltage of the RF trapping field to the ring electrodes. There are at least three problems with this '860 approach. First, the use of mass instability scanning to eject ions of mass less than m(p) suffers from poor mass resolution and thus results in significant loss in the intensity of the m(p) ion while attempting to completely move the m(p)-1 ion out of the stability region. Second, the stability boundary on the high side is flat so that this procedure also suffers significant loss of the m(p) ion when trying to eliminate the m(p)+1.
Finally, to use the '860 technique, it is essential to know the precise value of the trapping field operating on the ions in order to calculate the precise frequency to apply to the supplementary field. This precise frequency is difficult to know because of mechanical or electrical imperfections and because of space charge effects which act to significantly shift the stability region. The equation used to calculate the supplemental frequency which is given in the '860 patent is W=1/2.beta..sub.z W.sub.o, where W.sub.o is the frequency of the RF trap field.
The value .beta..sub.z is known to be defined by several approximating formulas, each of which are known to be accurate only for regions of the stability chart for lower values of the q.sub.z. Accordingly, it has become common to apply the supplemental frequency to eliminate the high m(p)+1 values at low values of q.sub.z parameter. In this low q.sub.z region, the relationship between the mass and resonant frequency is non-linear and the resolution at usual scan speed is poor. Furthermore, there is a limit to the maximum mass which can be ejected by this technique. To increase the value of the RF field beyond this value will also eject the parent ion of interest. To reach these higher mass value ions, the '860 patent adds an additional step of frequency scanning the supplemental frequency downward to low frequencies. This frequency scanning technique requires complex equipment and also introduces undesirable additional process time into the isolation process.
U.S. Pat. No. 4,762,545 discloses a technique called tailored excitation ion spectroscopy for employing Fourier synthesized excitation to create a time domain excitation waveform to cause tailored ejection of specific bands or ranges of ions. As pointed out in the '545 patent, the tailored FT method requires an extremely high power amplifier with high voltage output unless phase scrambling is employed. U.S. Pat. No. 4,945,234 discloses that phase scrambling distorts the excitation spectrum so that it is not possible to achieve arbitrary excitation frequency spectra at suitable low peak excitation voltages at the same time and that corrections are required for certain so called Gibbs oscillations. FT tailored excitation requires very expensive computational and RF synthesization equipment in order to be capable of tailoring to any desired frequency components.